A mechanism for the anomerization (1- epimerization) of alpha-D-glucose-6-phosphate by phosphoglucose isomerase has been postulated. Kinetic studies to define the rate determining step of this process and active-site alkylations to define the catalytic groups involved are proposed. Support for a regulatory significance to the observed specificities and anomerase activities will be sought in a study of liver aldolase specificity and possible control of phosphoglucose isomerase anomerase activity. The anomerase activity of yeast aldolase will be examined to determine the mechanism particularly in relation to the cleavage reaction.